This invention relates to electronic power supplies equipped with means to detect an AC power failure and which include circuitry to provide error-free switchover to a standby battery power source.
In microprocessor based electronic systems, as in other electronic circuits in which the main source of power is the AC power mains, a need often arises to provide emergency power for critical circuitry that must remain operational and/or store data when the AC power fails. This problem is particularly severe in some microprocessor systems, since the battery must provide power to time-keeping circuitry and preserve full memory storage for some minimum period of time which may be as long as a month or longer. Moreover, the battery must retain sufficient power reserve even after several years of storage in remotely installed, unattended systems. Therefore, if the battery is a primary cell (i.e., non-rechargeable) no drain current or battery external leakage should occur during the standby period.
It is also important that the power supply have circuitry to detect an AC power failure and to switch over to the battery with only negligible switching transients. Upon restoration of the AC power, switchover from the battery to the AC power supply must also occur in a transient-free manner. The AC fail signal should provide an unambiguous, error-free indication that an AC failure has occurred. The AC fail signal voltage must be maintained for a certain minimum interval. This permits the microprocessor, for example, to identify the signal as indicating an AC power failure and to initiate a power-down mode routine which turns off high power consuming circuitry. The microprocessor may also reduce the system clock frequency to further reduce standby battery power consumption.
The present invention provides an AC fail-detect circuit and battery switchover circuitry in accordance with the aforedescribed features.